What is the spacial use of friend function in c++

Question

actually I want to know why friend function

Answer 1

A friend function (or friend class) can be given access to the private members of a class by the class declaring it as a friend.

For example:
class A {
private:
friend void foo();
int a;
}

void foo() {
//foo can manipulate A::a, even though it is private
A x;
x.a = 10;
}

As the joke goes: "only C++ gives friends access to their private member"!